Android属性：所设属性值为何在重起后被清除

65 阅读 0 评论 43 点赞

我是靠谱客的博主老迟到自行车，最近开发中收集的这篇文章主要介绍Android属性：所设属性值为何在重起后被清除，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

问题：发现adb sehll setProp所设属性值在下次重起后被清除

adb shell setprop testing.mediascanner.skiplist /storage/sdcard1/test

结论：1.必须采用persist.开头的属性名才能永久保存。

2.如果具有root权限，可以直接编辑/system/build.prop并加入需要永久保存的属性

On system initialization, Android will allocates a block of shared memory for storing the properties. This is done in “init” daemon whose sourcecode is at: device/system/init. The “init” daemon will start a PropertyService.

The Property Service is running in the process of “init”daemon. Every client that wants to SET property needs to connect to theProperty Service and send message to Property Service. Property Servicewill update/create the property in shared memory. Any client that wants to GET property can read the property from the shared memory directly.This promotes the read performance.
Java API:

import android.os.SystemProperties;

public static String get(String key, String def) {}
public static void set(String key, String val) {}
The Native API:

int property_get(const char *key, char *value, const char *default_value);

int property_set(const char *key, const char *value);

bionic/libc/include/sys/system_properties.h
#define PROP_NAME_MAX 32
#define PROP_VALUE_MAX 92
以上定义可知属性最大长度为３２，属性值最大长度９２字节．

bionic/libc/include/sys/_system_properties.h
#define PROP_PATH_RAMDISK_DEFAULT "/default.prop"
#define PROP_PATH_SYSTEM_BUILD "/system/build.prop"
#define PROP_PATH_SYSTEM_DEFAULT "/system/default.prop"
#define PROP_PATH_LOCAL_OVERRIDE "/data/local.prop"

属性服务启动后会从系统文件中读取默认的属性，并写入共享内存中，以下4个文件为按顺序读取：
/default.prop
/system/build.prop
/system/default.prop
/data/local.prop

后读入的属性将覆盖前面读取的相同的属性。
system/core/init/property_service.c
void start_property_service(void)
{
int fd;

load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
#ifdef ALLOW_LOCAL_PROP_OVERRIDE
load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
#endif /* ALLOW_LOCAL_PROP_OVERRIDE */
/* Read persistent properties after all default values have been loaded. */
load_persistent_properties();

update_legacy_atvc_properties();

设置属性，ro.开头的属性将不能被更改属性值，persist.开头的属性会被永久纪录，其他属性值在重新开机后均将被丢弃：
int property_set(const char *name, const char *value)
{
if(namelen < 1) return -1;

pi = (prop_info*) __system_property_find(name);

if(pi != 0) {
/* ro.* properties may NEVER be modified once set */
if(!strncmp(name, "ro.", 3)) return -1;

pa = __system_property_area__;
update_prop_info(pi, value, valuelen);
pa->serial++;
__futex_wake(&pa->serial, INT32_MAX);
} else {
pa = __system_property_area__;
if(pa->count == PA_COUNT_MAX) return -1;

pi = pa_info_array + pa->count;
pi->serial = (valuelen << 24);
memcpy(pi->name, name, namelen + 1);
memcpy(pi->value, value, valuelen + 1);

pa->toc[pa->count] =
(namelen << 24) | (((unsigned) pi) - ((unsigned) pa));

pa->count++;
pa->serial++;
__futex_wake(&pa->serial, INT32_MAX);
}
    /* If name starts with "net." treat as a DNS property. */
    if (strncmp("net.", name, strlen("net.")) == 0)  {
        if (strcmp("net.change", name) == 0) {
            return 0;
        }
       /*
        * The 'net.change' property is a special property used track when any
        * 'net.*' property name is updated. It is _ONLY_ updated here. Its value
        * contains the last updated 'net.*' property.
        */
        property_set("net.change", name);
    } else if (persistent_properties_loaded &&
            strncmp("persist.", name, strlen("persist.")) == 0) {
        /*
         * Don't write properties to disk until after we have read all default properties
         * to prevent them from being overwritten by default values.
         */
        write_persistent_property(name, value);
    }
    property_changed(name, value);
    return 0;
}

当用户设置属性时，如果以属性名字以persist.开头，则会同时在/data/property目录下创建以该属性名字命名的文件，并写入属性值。

#define PERSISTENT_PROPERTY_DIR "/data/property"
static void write_persistent_property(const char *name, const char *value)
{
const char *tempPath = PERSISTENT_PROPERTY_DIR "/.temp";
char path[PATH_MAX];
int fd, length;

snprintf(path, sizeof(path), "%s/%s", PERSISTENT_PROPERTY_DIR, name);

fd = open(tempPath, O_WRONLY|O_CREAT|O_TRUNC, 0600);
if (fd < 0) {
ERROR("Unable to write persistent property to temp file %s errno: %dn", tempPath, errno);
return;
}
write(fd, value, strlen(value));
close(fd);

if (rename(tempPath, path)) {
unlink(tempPath);
ERROR("Unable to rename persistent property file %s to %sn", tempPath, path);
}
}

加载永久属性时，会读入在目录/data/property下所有名字以persist.开头的文件内容，作为该名字对应的属性值。

static void load_persistent_properties()
{
DIR* dir = opendir(PERSISTENT_PROPERTY_DIR);
...
if (dir) {
while ((entry = readdir(dir)) != NULL) {
if (strncmp("persist.", entry->d_name, strlen("persist.")))
continue;

ref:

1.Android Property Systemhttp://blog.csdn.net/magod/article/details/7255217